Secondary batteries are used in various types of applications such as electronic devices, elevators, transport devices such as vehicles and electric storage devices, and they have various sizes. In order to take full advantage of such secondary batteries, it is important to accurately estimate the SOC of the secondary batteries. When a significant error in the estimation of the SOC of the secondary battery is produced, since charging or discharging is performed beyond an allowable range, the secondary battery is disadvantageously expanded or increased in temperature. For example, a short circuit occurs, when the negative electrode collector foil of a secondary battery is electrically dissolved by over-discharging and precipitated again, whereby the negative electrode collector foil penetrates a separator. Consequently, a large amount of current is passed instantaneously to generate heat, and thus an electrolyte is evaporated. By over-charging, the electrolyte or the electrode material of the secondary battery is decomposed. When an internal pressure is increased by a gas produced by the heat generation or the decomposition reaction described above, the secondary battery is expanded. Hence, in order to take full advantage of a secondary battery while preventing the over-charging or the over-discharging of the secondary battery, it is necessary to accurately grasp the SOC of the secondary battery.
As a method for estimating the SOC of the secondary battery, a method using the OCV (Open Circuit Voltage) of the secondary battery is generally known. This method is a method for estimating the SOC by producing, in advance, a correlation curve of the SOC to the OCV of the secondary battery and introducing, into this correlation curve, an actually measured OCV which is measured during the use of the secondary battery.
However, depending on the constituent material of the secondary battery, a hysteresis may occur in which a correlation curve indicating a relationship between the SOC and the OCV differs between a charging process and a discharging process. When such a hysteresis occur, it is disadvantageously impossible to accurately estimate the SOC from the actually measured OCV.
Hence, in order to solve this problem, patent document 1 proposes a method for determining a correlation curve indicating a relationship between the SOC and the OCV for each SOC when switching from charging to discharging is performed, and measuring the SOC from an actually measured OCV based on this correlation curve.